The optimization of current communication networks, in particular of broadband user access networks—also known as access networks—aims to ensure access to broadband services—e.g. Video-On-Demand—for a large number of users at a reasonable price. One optimization possibility is to distribute the devices and resources provided by the user access networks (e.g. bandwidth and processing speed) over as many user connections as possible. A further cost optimization possibility is to arrange the intelligent functions required for current message services largely central in the user access network, so that, for example, the user access assemblies or central assemblies arranged in the communication network, or in the respective network devices, only require to be equipped with as little functionality as possible.
Further, the technological and economic costs involved in realizing network devices that can be arranged in current communication networks can be reduced by using technologies not developed especially for broadband user access networks but for the mass markets (e.g. personal computers). As an example of such a widespread and correspondingly developed technology one might mention the Ethernet standardized in accordance with Standard IEEE 802.3, which provides a frame or a packet oriented and connectionless transmission procedure. Ethernet based transmission systems are mainly used in local communication networks. In the case of network devices, such as for example multiplex devices (e.g. DSLAM, Digital Subscriber Line Access Multiplexer), that can be arranged in current communication networks, it is for example known, to convey message cells organized according to the asynchronous transmission mode-ATM—also referred to as ATM cells—via an Ethernet arranged locally in the network device between user access assemblies arranged in the network device and at least one central unit or assembly comprising the central functions of the network device. The Ethernet can be used both as “wiring” or “backplane” in an assembly rack to bridge smaller distances within the network device as well as an area-wide communication network to bridge larger distances.
The aim of future communication networks is to transmit ever greater data volumes at high transmission rates from and to the end user, whereby this is to be achieved in particular by using Ethernet based transmission procedures. To achieve a transition from ATM based systems whose realization involves great technical and hence financial costs to reasonably priced Ethernet based systems, it is especially necessary to run the ATM Service via system interfaces that are already based on Ethernet transmission technologies in current communication networks. Thereby, in particular the ATM transmission technology “Quality Of Service characteristics” must be maintained, whereby the available transmission capacity of current communication networks must be utilized to a satisfactory degree.
In the publication “ATM Forum, Technical Committee, Frame-based ATM Transport over Ethernet (FATE), AF-FBATM-0139.00, Feb. 2000” there is, for example, the description of a method by which ATM cells are transmitted via a communication network set up as an Ethernet. A disadvantage of the method described is that it is limited to ATM connections of the type AAL5. Thus this method can only be used to a limited extent in current and future user access networks, as in these networks, when current data and communication services are realized, ATM cells of all AAL types are transmitted, especially the type AAL1 and AAL2. A further disadvantage of this method is that just the user information in the respective ATM cells to be transmitted via the Ethernet—i.e. transport by AAL5 SSCS-PDUs—is added to the respective user data field of the Ethernet frame, which makes it necessary for the ATM cells that are to be transmitted to undergo a pre-processing procedure involving a technological input. This approach to a solution cannot be used in current user access networks as the AAL5 termination occurs with the user—i.e. at the CPE, Customer Premises Equipment—and the transmission system for the transmission of AAL5 information should be transparent. With the known method, a transparent transmission of the ATM cells via the Ethernet is not possible.
In addition, with the known method an own Ethernet frame is used to the disadvantage of each virtual connection—PVC, Permanent Virtual Connection—set up within the ATM communication network. Depending on the length of the frame, there is a delay for individual ATM cells, as incoming cells must be held in intermediate storage until the Ethernet frame is full. This is especially unfavorable for real time applications. An obvious alternative of adding only one single ATM cell into each Ethernet frame results, however, in the transmission resources provided by the communication network being poorly utilized.